Thermal barrier coating for a superalloy article

ABSTRACT

A coated article includes a superalloy substrate, an intermediate bond coat and a thermal barrier coating. The bond coat may include a platinum aluminide layer underlying a thin oxide layer. The thin oxide layer may include alumina. The coated article has high strength and durability at high temperatures over extended periods of time and thus is especially useful in the form of, e.g., a turbine blade or turbine vane.

This is a Division of application Ser. No. 08/569,480 filed Dec. 8,1995, now U.S. Pat. No. 5,645,893.

FIELD OF THE INVENTION

The present invention relates to a thermal barrier coating applied tothe surface of a superalloy article, e.g. a gas turbine engine turbineblade, and to a method of applying the coating.

BACKGROUND

The constant demand for increased operating temperatures in gas turbineengines was initially met by air cooling of the turbine blades anddevelopment of superalloys from which to manufacture the blades, both ofwhich extended their service lives. Further temperature increasesnecessitated the development of ceramic coating materials with which toinsulate the turbine blades from the heat contained in the gasesdischarged from the combustion chambers; again turbine operating lifewas extended. However, the amount of life extension was limited becausethe coatings suffered from inadequate adhesion to the superalloysubstrates, one reason for this being the disparity of coefficients ofthermal expansion between the superalloy substrate and the ceramiccoating. Coating adhesion was improved by the development of varioustypes of aluminum-containing alloy bond coats which were thermallysprayed or otherwise applied to the superalloy substrate before theapplication of the ceramic coating. Such bond coats are typically of theso-called aluminide (diffusion) or "MCrAlY" types, where M signifies oneor more of cobalt, nickel and iron.

Use of bond coats has been successful in preventing extensive spallationof thermal barrier coatings during service, but localized spallation ofthe ceramic still occurs where the adhesion fails between the bond coatand the ceramic layer. This exposes the bond coat to the full heat ofthe combustion gases, leading to premature failure of the turbine blade.

SUMMARY OF THE INVENTION

Thus one object of the present invention is to modify the bond coat toprovide a thermal barrier coating which is less prone to localizedfailure and more suitable for long term adhesion to a superalloysubstrate.

A further object is to provide a method of applying a thermal barriercoating to a superalloy article so as to achieve improved adhesionthereto.

According to one aspect of the present invention, a multi-layer thermalbarrier coating for a superalloy substrate includes: analuminum-containing bond coat on the substrate; a thin oxide layer onthe bond coat; and an outer ceramic insulating layer on the oxide layer;the bond coat being provided with a platinum-group metal enriched outerlayer having a surface layer predominantly comprising at least onealuminide of the platinum-group metals. The multi-layer thermal barriercoating on the substrate thus forms a completed multi-layer thermalbarrier coated article, and the completed coated article has yet to besubjected to thermal stress.

According to one aspect of the present invention, a multi-layer thermalbarrier coating for a superalloy substrate comprises a bond coating onthe superalloy substrate, the bond coating comprising an aluminumcontaining alloy coating and a coating predominantly comprising at leastone aluminide of the platinum-group metals, the aluminum containingalloy coating overlying the superalloy substrate, the coating of atleast one aluminide of the platinum-group metals overlying the aluminumcontaining alloy coating, a thin oxide layer on the bond coating, theoxide layer overlying the coating of at least one aluminide of theplatinum-group metals and an insulating ceramic coating on the oxidelayer, the bond coating comprises a platinum-group metal enrichedaluminum containing alloy layer between the aluminum containing alloycoating and the coating of at least one aluminide of the platinum-groupmetals, the platinum-group metal enriched aluminum containing alloylayer and the coating of at least one aluminide of the platinum-groupmetals reduce migration of transition metal elements through the bondcoating to the ceramic coating, the thin adherent oxide layer comprisesalumina without other spinels in amounts sufficient to substantiallydisrupt the alumina lattice structure. By an "aluminide of theplatinum-group metals" is meant any compound formed within the aluminumv. platinum-group metals phase diagrams, including alloys formed byincorporation of other metallic elements.

According to a further aspect of the present invention, a method ofapplying a multi-layer thermal barrier coating to a superalloy articlecomprises the steps of: applying an aluminum-containing alloy bond coatto the superalloy article; applying a thin layer of a platinum-groupmetal to the bond coat; heat treating the article to diffuse theplatinum-group metal into the bond coat and thereby create aplatinum-group metal enriched outer layer of the bond coat having asurface layer predominantly comprising at least one aluminide of theplatinum-group metals; creating a thin adherent layer of oxide on thealuminide layer, and applying a ceramic layer to the oxide layer,preferably by a physical vapour deposition process.

According to another aspect of the present invention, a method ofapplying a multi-layer thermal barrier coating to a superalloy articlecomprises the steps of: applying an aluminum containing alloy coating tothe superalloy article, forming a coating predominantly comprising atleast one aluminide of the platinum-group metals on the aluminumcontaining alloy coating, creating a thin adherent layer of oxide on thecoating of at least one aluminide of the platinum-group metals andapplying an insulating ceramic coating to the oxide layer, a layer ofplatinum-group metal is applied to the aluminum containing alloycoating, the superalloy article is heat treated to diffuse theplatinum-group metal into the aluminum containing alloy coating tothereby create the coating of at least one aluminide of theplatinum-group metals and a platinum-group metal enriched aluminumcontaining alloy layer between the aluminum containing alloy coating andthe coating of at least one aluminide of the platinum-group metals, athin adherent layer of alumina without other spinels in amountssufficient to substantially disrupt the alumina lattice structure iscreated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An advance over prior art coatings is evident in that the platinum-groupmetal aluminide surface coating facilitates the creation of an oxidelayer comprising at least 70% by volume of alumina, preferably at least90 vol.% alumina, most preferably at least 95 vol.% alumina.Qualitatively, utilizing appropriate material specifications and processsteps as taught in this specification, it is believed that the presentinvention enables the creation of an oxide layer comprising aluminawithout other spinels in amounts sufficient to substantially disrupt thealumina lattice structure.

For the purposes of the present specification, a spinel is defined as anoxide having the general formula M₂ O₃, where M signifies a transitionmetal.

Preferably, the thickness of the oxide layer as produced by the aboveprocess is less than one micron.

The aluminum-containing alloy bond coating may be a nickel or cobaltaluminide, but an MCrAlY alloy is preferred, where M is at least one ofNi, Co and Fe. The bond coat's aluminum content will depend upon thetype of bond coating alloy chosen for use with the invention, being aminimum of about 5% by weight for an MCrAlY alloy bond coating and amaximum of about 40% by weight for an aluminide bond coating.

Preferably, in the finished article, the outer layer of the bond coat isenriched with platinum and in this case the aluminide surface coatingpredominantly comprises platinum aluminide.

We believe that such a platinum aluminide surface coating will containat least 25 wt.% platinum, preferably at least 40 wt.% and optimally atleast 50 wt.% platinum, with aluminum levels of at least 8 wt.%,preferably at least 10 wt.%.

To produce a platinum enriched aluminum containing alloy layer with analuminide surface coating predominantly comprising platinum aluminide,the thickness of the layer of platinum as applied before diffusion ispreferably greater than 3 μm, more preferably at least 5 μm.

The diffusion heat treatment is preferably carried out for about onehour at a temperature in the range 1000-1200° C., preferably 1100-1200°C., depending upon the composition of the superalloy substrate. Aftercleaning off any diffusion residues from the surface of the platinizedbond coating, the article receives its thin adherent layer of oxide andits ceramic top coating.

The thin adherent layer of oxide is preferably created by heating theplatinum-group metal aluminide coating in an oxygen-containingatmosphere.

Conveniently for the creation of the thin adherent oxide layer, weprefer to use electron beam physical vapour deposition (EBPVD) to applythe ceramic layer. In the preferred EBPVD process, the article ispreheated to a temperature in the range 900-1150° C. in a vacuum, say ata pressure of about 10⁻⁵ Torr. A preferred preheat temperature is about1000° C.

The EBPVD ceramic coating process, using yttria stabilized zirconia orother oxide ceramic, involves evaporation of the ceramic by the electronbeam and consequent liberation of oxygen by dissociation of the ceramic.We also prefer to add oxygen to the coating chamber deliberately at thisstage to encourage stoichiometric re-formation of the ceramic on thearticle being coated, though this is not always considered essential bysome sectors of the industry. Hence, in our preferred process, oxygen isinevitably present in the atmosphere of the coating chamber duringcoating by EBPVD and reacts with the preferred platinum aluminidesurface coating of the bond coating, forming the thin adherent oxidelayer mentioned above, a typical thickness of the oxide layer being lessthan 1 μm. The ceramic coating adheres well to the oxide layer, thelatter effectively becoming a bonding layer. It is due to the presenceof the preferred platinum enriched aluminum containing alloy layer andplatinum aluminide coating of the bond coating that the oxide layerformed during the preheating step can comprise greater than 95 vol.%alumina (Al₂ O₃).

During subsequent aging of the coating in service, the oxide layer growsgradually in thickness due to diffusion of oxygen through the coatingfrom the environment. However, it is found that the preferred platinumenriched aluminum containing alloy layer of the bond coating, andparticularly the preferred platinum aluminide surface coating, restrictsthe rate of growth of the oxide layer and promotes the evenly spreadproduction of alumina therein, rather than the production of otherspinels having different crystal lattice structures which are known tohave higher growth rates and therefore reduce adhesion of the ceramiccoating thereto.

Use of EBPVD in application of the ceramic coating can be with orwithout plasma assistance. However, as disclosed in copending publishedInternational patent application No. WO93/18199, both EBPVD techniquescan be used alternately, which enables control of the columnar coatingstructure so as to ensure high strength with decreased thermalconductivity. For further detail concerning such coatings and theprocesses of applying them, reference should be made to the above patentpublication, which is hereby incorporated by reference.

Further aspects of the invention will be apparent from a perusal of thefollowing description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example and with referenceto the accompanying drawings, in which:

FIGS. 1A and 1B are cross-sectional diagrammatic views through ametallic article having a prior art thermal barrier coating appliedthereto;

FIGS. 2A and 2B are cross-sectional diagrammatic views through ametallic article having a thermal barrier coating in accordance with thepresent invention;

FIG. 3 is a bar chart showing the results of tests of relativeperformance of four different coating types;

FIG. 4 is a bar chart showing the influence of diffusion heat treatmenttemperatures on performance of coatings in accordance with theinvention;

FIG. 5 is a cross-sectional diagrammatic view through a metallic articleshowing the thermal barrier coating in more detail;

FIG. 6 is a bar chart showing the influence of the method of depositionof the MCrAlY coating on the performance of coatings according to thepresent invention.

FIG. 7 is a bar chart showing the influence of different metals in theplatinum-group of metals on the performance of coatings according to thepresent invention; and

FIG. 8 is a bar chart comparing relative performance of four differentcoating types.

Referring first to FIG. 1A, illustrating the state of the art, there isshown part of a superalloy article 10 provided with a multi-layerthermal barrier coating indicated generally by numeral 11. It is shownin the "as manufactured" condition.

Coating 11 is constituted generally as follows. It comprises aplasma-sprayed and diffusion heat treated MCrAlY alloy bond coat layer12 and a columnar layer of yttria-stabilized zirconia ceramic 14 appliedthereover by a known EBPVD process.

The microstructure of the MCrAlY bond coat 12 broadly comprises threephases, these being an alpha phase, a beta phase, and a small amount ofan yttrium-rich phase. The alpha phase comprises a solid solution ofnickel, cobalt, chromium, yttrium and aluminum, with minor amounts ofother elements which have migrated from the substrate. The beta phasecomprises an aluminide of cobalt, nickel and aluminum, with chromium andother metallic elements dissolved in the aluminide up to certainsolubility limits.

Importantly, at the interface between the layers 12 and 14 there is athin oxide layer 16, produced as a consequence of the EBPVD coatingprocess. As in the other Figures, the thickness of the oxide layer 16 inFIG. 1A is exaggerated relative to the other layers of the coating 11.

EXAMPLE 1

A batch of specimens as illustrated in FIG. 1A were produced. In thisexample, as in the further Examples 2 to 6 described below, the article10, which forms the substrate for the coating 11, was made of anickel-based superalloy called MAR-M 002, a trade name of the MartinMarietta Corporation, of Bethesda, Md., U.S.A. Its nominal compositionis given in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        ELEMENT       WEIGHT %                                                        ______________________________________                                        Tungsten      10                                                              Cobalt        10                                                              Chromium      9                                                               Aluminum      5.5                                                             Tantalum      2.5                                                             Titanium      1.5                                                             Hafnium       1.5                                                             Carbon        0.15                                                            Nickel        Balance                                                         ______________________________________                                    

In this example, as in the further Examples 2 to 6 described below, thealuminum-containing bond coat alloy 12 was of the MCrAlY type. Itscomposition is given in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        ELEMENT   WEIGHT % MINIMUM                                                                             WEIGHT % MAXIMUM                                     ______________________________________                                        Nickel    31.0           33.000                                               Chromium  20.0           22.000                                               Aluminum  7.0            9.000                                                Yttrium   0.35           0.650                                                Carbon    0.00           0.025                                                Oxygen    0.00           0.050                                                Nitrogen  0.00           0.010                                                Hydrogen  0.00           0.010                                                Other Elements                                                                          0.00           0.500                                                in total                                                                      Cobalt    Balance                                                             ______________________________________                                    

The alloy specified in Table 2 is available from Praxair SurfaceTechnologies, Inc., (formerly Union Carbide Coating ServiceCorporation), of Indianapolis, U.S.A., under the trade name LCO22.

The bond coat alloy of Table 2 is initially in powder form, its meshsize range being:

<270 mesh up to 100% by weight

>325 mesh maximum 1% by weight

The powder size range in μm is:

<5 μm maximum 5% by weight

<10 μm 10-15% by weight

<20 μm 35-55% by weight

To produce the prior art coating 11, the following procedure wasfollowed.

After thorough preparation and cleaning of the surface of the article 10by grit blasting with fine alumina grit and degreasing, the bond coatalloy powder mix was thermally sprayed in known manner onto the surfaceby use of a plasma gun in an evacuated chamber.

To ensure bonding of the MCrAlY coating to the substrate, the sprayedarticle was diffusion heat treated at 1100° C. for one hour. Thisproduced the three-phase alloy microstructure noted above.

After removal of diffusion residues by grit blasting and degreasing, alayer of a ceramic 14 consisting of partially stabilized zirconia (inthis case, zirconia containing 8% by weight of yttria) was applied byelectron beam physical vapour deposition (EBPVD). This coating isavailable from Chromalloy Gas Turbine Corporation of Delaware, U.S.A.

For the EBPVD process, the article was first held in a preheatingchamber and preheated to a temperature of about 1000° C. at a pressureof 10⁻⁵ Torr. It was then immediately transferred to an electron beamcoating chamber, where it continued to be held for coating at 1000° C.,at a pressure of 10⁻² to 10⁻³ Torr, in an atmosphere consisting of argonand oxygen.

It should be noted that in the present and following Examples 1 to 4,some of the free oxygen in the coating chamber's atmosphere results fromthe dissociation of ZrO₂ as it is evaporated by the electron beam in thecoating chamber. The dissociated constituents of the ceramic recombinewith each other as the vapour is deposited on the article. However,unless assisted, this recombination tends to be incomplete, i.e., theoxygen binds to the zirconium in sub-stoichiometric proportions,resulting in a deficiency of oxygen in the ceramic and free oxygen inthe atmosphere of the coating chamber. Recombination of the ceramic instoichiometric proportions is assisted by providing an excess of oxygen,thereby further adding to the amount of oxygen in the coating chamber.

The presence of oxygen at an elevated temperature during the EBPVDcoating process made it inevitable that a thin oxide layer formed on thesurface of the bond coat. The oxide layer was covered by the ceramiclayer and became the interface oxide layer 16 mentioned above. As shownin Example 5, the oxide layer so produced was a mixture of alumina,chromia and other spinels.

Some specimens were subjected to a standardized adhesion test in whichthe strength of the bond between the ceramic layer and the bond coat wasdetermined. On average, it was found that the critical load, beyondwhich the ceramic would break away from the bond coat, was about 55Newtons.

The remaining specimens were then subjected to an ageing process tosimulate a period of service in the turbine of a gas turbine engine.FIG. 1B illustrates one of the specimens after it had been subjected toageing by heating in air to a temperature of 1150° C. for 100 hours.

The oxide layer 16', initially produced during the coating process aslayer 16 in FIG. 1A, had thickened and changed in character during theageing process, as will be explained below.

Several specimens aged like that in FIG. 1B were assessed as before todetermine the strength of the bond between the ceramic layer and thebond coat. On average, it was found that the critical load was now lessthan 5 Newtons.

The following explanation for this large decrease in coating adhesivestrength is tentatively advanced.

Firstly, alumina, chromia and other spinels have differing crystallattice structures. The oxide layer apparently becomes more incoherentwith ageing, and therefore weaker and less adherent to the adjacentlayers. It is believed that this is due to migration of transitionmetals other than aluminum from the superalloy substrate 10 and the bondcoat 12 into the base of the ceramic top coat 14, thereby promotinggrowth of several spinels with incompatible lattice structures as oxygenfrom the environment diffuses through the ceramic top coat 14.

Secondly, the general thickening of the oxide layer 16' producesincreased inherent stress at the interface between the bond coat and theceramic layer, weakening overall adhesion.

Thirdly, alumina has a lower growth rate than the other spinels. Theeffect of the higher growth rate of these other spinels is particularlyevident in the region 18, where localized portions of the developedoxide layer 16' have grown to greatly exceed the thickness of the layerelsewhere. It has been found that during service of coatings of theabove type, spalling of the coating tends to occur wherever thethickness of the oxide layer 16 exceeds about 5 μm.

EXAMPLE 2

Referring now to FIG. 2A, a batch of specimens prepared in accordancewith the present invention comprised a superalloy article 10 providedwith a multi-layer thermal barrier coating 11a. Coating 11a comprised abond coat 12a, which was initially the same as the bond coat 12 ofExample 1, and a columnar ceramic top coat 14, as in Example 1. A thinoxide layer 16a was also present at the interface between the bond coatand the ceramic coat, again produced during the EBPVD ceramic coatingprocess.

However, the finished structure of coating 11a differed from Example 1,inter alia, in that the bond coat 12a was provided with aplatinum-enriched outer layer 20 underneath the oxide layer 16a.Furthermore, when a specimen was subjected to electron probemicroanalysis, the platinum enriched layer 20, particularly an outerzone 21 of layer 20 (say the top 15-20 μm) was notable for containing anenhanced proportion of aluminum. Both zone 21 and, to a lesser degree,the rest of the platinum enriched layer 20, contained depressedproportions of nickel, cobalt and chromium relative to an underlyinglayer 13 of the bond coat 12a. Layer 13, particularly its outer part,also exhibited lower aluminum content than was present in the initialMCrAlY alloy, due to some of the aluminum having migrated towards theouter parts of the bond coat 12a.

In a traverse of the bond coat 12a in the outward direction, themicrostructure of the layer 13 appeared similar to the MCrAlY alloy bondcoat 12 in Example 1, exhibiting alpha, beta and yttrium-rich phases.However, the microstructure of platinum enriched layer 20 exhibited arapid change away from the MCrAlY alloy phases, due to the above-notedchanges in elemental proportions compared with layer 13, while in anoutermost surface layer 22 of the bond coat, the microstructure wascharacteristic of platinum aluminide, layer 22 being several micronsthick. Also notable in the outermost several microns of the bond coatwas the presence of yttrium which had migrated from the underlyinglayer. Yttrium is known to promote adherence of the oxide layer 16a.

Furthermore, as separately shown in Example 6, the oxide layer 16aconsisted wholly or almost wholly of alumina, with much smaller ornegligible amounts of the other spinels which had been produced inExample 1. Thickness of the alumina layer 16a after application of theceramic coating was found to be less than 1 μm.

Production of such a specimen followed the same process steps as forExample 1, except in the following particulars.

After application, diffusion and cleaning of the bond coat 12a, a layerof platinum of substantially constant thickness was applied, thethickness being about 8 μm. However, in performing the invention thethickness could vary upwards of about 3 μm, depending on a number offactors, such as substrate and bond coat materials used, diffusiontemperatures and service conditions. The importance of these factors inrelation to platinum coating thickness can be determined by experiment.The platinum layer was applied by electroplating, but any other meanscould be used which will achieve a sufficient substantially uniformthickness without detriment to the material's properties.

A further diffusion heat treatment step was then effected so as to causethe platinum layer to diffuse into the bond coat. This provided theplatinum-enriched layer 20, with the outer layers 21 and 22 having theparticular characteristics mentioned above.

Diffusion was achieved by heating the article to a temperature of 1190°C. In a vacuum chamber and holding that temperature for one hour. Inperforming the invention a range of heat treatment temperatures could beused from 1000° C. to 1200° C. inclusive, according to the solutiontreatment temperature normally used for the superalloy substrate 10. Inthe present example, 1190° C. is higher than the accepted solutiontreatment temperature for MAR-M 002, but was utilized as one of a rangeof diffusion treatments as explained later. Although different diffusiontimes could be used, it was judged that one hour was sufficient at thisrange of temperatures for the platinum to be properly combined with thebond coat without prematurely ageing the bond coat and the underlyingsubstrate.

After heat treatment the surface was grit blasted with dry Al₂ O₃ powderof a 120-220 μm particle size to remove any diffusion residues. Theceramic top coat 14 was then applied as in Example 1.

Several specimens like that in FIG. 2A were tested in the same way as inExample 1 to determine the strength of the bond between the ceramic coatand the bond coat. On average, it was found that the critical load,beyond which the ceramic would break away from the bond coat, was about100 Newtons, nearly a two-fold improvement over Example 1. This isbelieved to result from the better coherence of the oxide layer 16a asinitially produced during the ceramic coating process. Evidently, thepresence of the platinum enriched layer in the outer part of the bondcoat prevented migration of transition elements through the bond coatinto the base of the ceramic layer during the coating process.Furthermore, it is believed that the presence of platinum, particularlythe platinum aluminide surface layer 22, promoted the inward growth ofthe alumina during the ceramic coating process, instead of a more rapidoutward oxide growth into the base of the ceramic layer.

FIG. 2B illustrates the same coating as FIG. 2A, but after the article10 had been subjected to the same ageing process as was mentioned forExample 1, i.e., heating in air to a temperature of 1100° C. for 100hours. The oxide layer 16a', initially produced during the coatingprocess as oxide layer 16a, has thickened somewhat, but not as much asin Example 1.

Several specimens like that in FIG. 2B were assessed as before todetermine the strength of the bond between he ceramic layer and the bondcoat after ageing. On average, it was found that the critical load wasstill nearly 50 Newtons, on the order of a twenty-fold increase comparedwith the coating of Example 1 after ageing.

The following explanation for this large increase in coating adhesivestrength is tentatively advanced.

Firstly, the oxide layer had apparently retained much of its initialcoherence despite ageing, and had therefore retained more strength andadherence to the adjacent layers. As mentioned above in respect of theceramic coating step, this is believed due to the platinum-enrichedlayer 20 acting as a barrier to migration of transition metals from thesuperalloy substrate 10 and the bond coat 12a into the base of theceramic top coat 14, thereby preventing growth of spinels with latticestructures incompatible with alumina.

Secondly, the thickening of the oxide layer during ageing was both lessin magnitude and more even in distribution than in Example 1. Thisproduced less inherent stress at the interface between the bond coat andthe ceramic layer. This is believed due to the coherent alumina layeracting as a more uniform barrier to diffusion of oxygen through theceramic top coat into the bond coat.

Formation of the platinum-enriched layer 20 is believed to proceed asfollows.

The MCrAlY alloy bond coat contains nickel-cobalt aluminide (NiCoAl).During the platinizing diffusion heat treatment, this aluminide isbroken down due to aluminum's greater chemical affinity for platinum.The freed aluminum therefore rapidly migrates towards the platinum andcreates the platinum aluminide surface layer 22 and the notablydepressed nickel, cobalt and chromium content in zone 21 relative to therest of the platinum enriched layer 20 and the MCrAlY alloy layer 13.Although the platinum aluminide is concentrated near the surface of thebond coat in layer 22, it is also present to a decreasing extentthroughout the depth of layer 20. All or most of the platinum iscombined with aluminum in this way.

During ageing, the alumina layer is believed to grow as follows.

During ageing, oxygen diffusing through the ceramic layer 14 and theexisting alumina layer 16a, strips the aluminum from the platinum at thesurface of the platinum aluminide layer 22 and preferentially combineswith it, so maintaining gradual inward growth of the substantially purealumina layer. The freed platinum then attracts further aluminum, whichmigrates from deeper in the platinum enriched layer 20 and the MCrAlYalloy layer 13, so maintaining a supply of aluminum at the surface ofthe layer 22 for combination with further oxygen. This maintains thecoherence of the oxide layer during the ageing process.

EXAMPLE 3

A further batch of specimens (not illustrated) was prepared with amulti-layer thermal barrier coating again comprising an MCrAlY bond coatand a columnar ceramic top coat. As before, a thin oxide layer was alsopresent at the interface between the bond coat and the ceramic coat,again produced during the EBPVD ceramic coating process.

The coating differed from Examples 1 and 2 in that the superalloysubstrate was provided with a platinum-enriched surface layer directlyunderneath the bond coat.

Production of such a specimen followed the same process steps as forExample 1, except that before application of the bond coat, a layer ofplatinum having a substantially constant thickness of about 8 μm wasapplied to the superalloy substrate by electroplating and was thendiffused into the substrate to provide the platinum enriched layermentioned above.

As for Example 2, platinizing diffusion was achieved by heating thearticle to a temperature of 1190° C. In a vacuum chamber and holdingthat temperature for one hour. After heat treatment the surface was gritblasted to remove diffusion residues. The bond coat and ceramic top coatwere then applied as in Example 1.

Several such specimens were tested in the same way as in Example 1 todetermine the strength of the bond between the ceramic coat and the bondcoat. On average, it was found that the critical load, beyond which theceramic would break away from the bond coat, was about 85 Newtons, animprovement over Example 1, but less than achieved for Example 2. Thisintermediate result is believed to reflect a coherence of the oxidelayer intermediate to that evidenced in Examples 1 and 2. Evidently, theplatinum enriched layer in the outer part of the superalloy substrateprevented migration of substrate transition elements through the bondcoat into the base of the ceramic layer during the ceramic coatingprocess, but could not of course prevent migration of cobalt, nickel andchromium from the bond coat towards the ceramic layer.

After several specimens had been subjected to the same ageing process aswas mentioned for Examples 1 and 2, they were assessed as before todetermine the strength of the bond between the ceramic layer and thebond coat after ageing. On average, it was found that the critical loadshowed no improvement over Example 1. This result implies that similardeterioration of the oxide layer occurred, allowing increasing amountsof oxygen across the oxide layer to combine with migrating elements fromthe bond coat.

EXAMPLE 4

Another batch of specimens (not illustrated) was prepared using aprocess which differed from Example 2 only in that the layer of platinumelectroplated onto the 5 bond coat was about 3 μm thick instead of 8 μmthick. Diffusion heat treatment was the same as for Example 2 to providea platinum-enriched outer layer for the bond coat.

Specimens were tested before and after ageing in the same way as inExamples 1 to 3.

Before ageing, the average critical load was about 70 Newtons, animprovement over Example 1, but less than achieved for Example 3. Thisresult is believed to reflect inadequate thickness of the depositedplatinum layer and consequent inadequate platinum enrichment of theoutermost layer of the bond coat. Evidently, the platinum enrichment wasinsufficient to prevent migration of transition elements through thebond coat into the base of the ceramic layer during the coating process.After ageing, the average critical load showed no significantimprovement over Example 1. This result again implies that insufficientplatinum enrichment had occurred.

To achieve the full benefits of the invention using platinized MCrAlYbond coats, we prefer that the thickness of the electroplated platinumlayer be significantly in excess of 3 μm, more preferably at least 5 μm,most preferably 8 μm, as in Example 2. 8 μm is not an upper limit, butwe prefer not to apply platinum to a much greater thickness than thisdue to the expense of platinum as a raw material and the probabilitythat longer diffusion heat treatment times would be needed to obtainrelatively small increases in coating adhesion.

Referring now to FIG. 3, there are shown comparative adhesion strengthsfor coatings according to Examples 1 to 4 when subjected to a range ofageing treatments. The results for no ageing and for ageing at 1150° C.for 100 hours have already been quoted above in respect of each Example.Also shown in FIG. 3 are the results for ageing some specimens for thesame period but at the lower temperatures of 1050° C. and 1100° C.

The chart shows that increased severity of ageing up to a temperature of1100° C. reduces coating adhesive strength by up to about half of itsoriginal value, this weakening being least pronounced for Example 1,though this example's adhesive strength was lowest to begin with.However, for Examples 1, 3 and 4, further increasing the severity ofageing up to a temperature of 1150° C. reduces coating adhesive strengthto almost zero, whereas for Example 2, the same increase in ageingtemperature produces no further substantial decline in adhesivestrength, within the limits of experimental error, thereby illustratingthe utility of the present invention.

Referring now to FIG. 4, there are shown comparative adhesion strengthsfor coating specimens according to Examples 1 and 2 when subjected to arange of platinizing diffusion heat treatments with and withoutsubsequent ageing. The platinizing diffusion heat treatments range fromnone for Example 1 (since it has no platinum enriched layer) throughtreatments at 1100° C., 1150° C. and 1190° C. for Example 2, the periodsof heat treatment all being one hour. The results for these specimenswithout ageing and for ageing at 1100° C. for 100 hours are shownside-by-side.

The chart shows that increased platinizing diffusion temperature forcoatings in accordance with the invention leads to increased coatingadhesive strength both before and after ageing. Although the highestdiffusion temperature of 1190° C. produced the most pronouncedstrengthening effect in specimens according to Example 2, even afterageing, this diffusion treatment would not in practice be preferredbecause it exceeds the normal solution treatment for MAR-M 002 alloy,which is 1150° C. for one hour. The normal solution heat treatmentcoincides with the next most severe platinizing diffusion heat treatmentshown in FIG. 4, which is therefore preferred for this particularsuperalloy. Diffusion at 1150° C. still yields a large increase incoating bond strength after ageing compared with the prior art. Asbefore, the prior art Example 1 was initially the weakest coating andageing reduced its strength to almost zero.

To enable a comparison to be made between the composition of an oxidelayer as produced by a prior art process, and the composition of anoxide layer as produced by a process according to the invention, furtheranalysis was conducted as described below in Examples 5 and 6.

EXAMPLE 5

Production of a specimen for analysis followed the same process steps asfor Example 1, except that the final 10 columnar ceramic top coat wasnot applied. Instead, the oxide layer on the MCrAlY alloy coating wasdeveloped using a simulation of the ceramic coating process conditionsexplained in Example 1, but without EBPVD deposition of the yttriastabilized zirconia being initiated. Consequently, after production ofthe oxide layer, it could be analyzed using an X-ray diffractiontechnique. This showed that the oxide layer which had been developed onthe MCrAlY bond coat comprised a mixture of approximately 65% by volumeof alumina, with 35 vol.% nickel and chromium oxides having the generalformula M₂ O₃.

EXAMPLE 6

Production of a specimen followed the same process steps as for Example2, except that the final columnar ceramic top coat was not applied. Asin Example 5, the ceramic coating process conditions were simulated soas to develop the oxide layer without covering it with the ceramiccoating. When an X-ray diffraction analysis was performed on the oxidelayer, it was found to comprise 100 vol.% alumina, with an experimentalerror of about -5 vol.%. Evidently, the process steps of Example 2 hadenabled the creation of an oxide layer comprising alumina without otherspinels in amounts sufficient to substantially disrupt the aluminalattice structure.

As already mentioned, the notable increase in adhesion which is providedby thermal barrier coatings in accordance with the invention, such asthat described herein with reference to FIG. 2 and Example 2, isbelieved due to the oxide layer 16a being composed of alumina withoutlarge amounts of other spinels, even after ageing has occurred. Whilstgrowth of the oxide layer 16a during service is still inevitable, as itwas between the ceramic coat and bond coat of the prior art, the addedoxide is now substantially all Al₂ O₃ rather than the mixed spinels ofthe prior art.

The attributes of a good bond coating for good adhesion of a thermalbarrier coating are that the bond coating has the ability to prevent, orreduce, the migration of transition metal elements to the ceramicthermal barrier coating. The migration of transition metal elements ispreferably blocked by a continuous layer in the bond coating, or slowedby formation of stable compounds by a layer in the bond coating. Thisattribute enables the resulting thermally grown oxide formed on the bondcoating to be very pure alumina. The bond coating is preferably stableto ageing at high temperatures so that it still prevents, or reduces,the migration of transition metal elements to ensure that any furthergrowth of the oxide on the bond coating is by formation of alumina.These attributes are facilitated by the formation of a stable layerclose to the thermally grown oxide interface between the bond coatingand the ceramic thermal barrier coating.

The following advantages accrue from practice of the invention:

A. The growth of the oxide layer is slowed, resulting in the formationof a thinner layer over longer periods of time.

B. The oxide layer remains a substantially even thickness during ageing,thus providing more constant adhesive quality over the whole interfaceof the ceramic with the oxide.

C. At least in the case of vacuum plasma sprayed MCrAlY coatings whichhave been platinized, adhesion of the oxide layer seems to be enhancedby the presence of yttrium in the platinum aluminide layer.

D. The platinum enriched layer of the bond coat restricts the migrationof transition metals through the bond coat into the ceramic.

E. The true capability of the thermal barrier coating to act as athermal barrier is enabled for longer periods of time than heretofore,through the achievement of considerable delay of the onset of spalling.It follows that where the article is a superalloy turbine blade, itsoperating life is enhanced.

The additional FIG. 5 shows the multi-layer thermal barrier coating 11ashown in FIGS. 2A and 2B in more detail. The thermal barrier coating 11acomprises a bond coat 12a, and oxide layer 16a and a columnar ceramictop coat 14. The bond coat 12a comprises a MCrAlY alloy layer 13overlying the superalloy article 10, and a platinum enriched alloy layer20 overlying the MCrAlY alloy layer 13. The oxide layer 16a overlies theplatinum enriched alloy layer 20 and the columnar ceramic top coat 14overlies the oxide layer 16a.

As mentioned previously the platinum enriched alloy layer 20 contains anenhanced proportion of aluminum and depressed proportions of nickel,cobalt and chromium relative to the MCrAlY alloy layer 13. The platinumenriched alloy layer 20 has an outer zone 21 which is particularly richin aluminum. The outer zone 21 comprises an outermost surface layer 22which is characteristic of platinum aluminide, we have found that thisplatinum aluminide is a previously unknown form of platinum aluminidewhich we have termed the "P" phase. The "P" phase for example has acomposition of 53 wt% Pt, 19.5 wt% Ni, 12 wt% Al, 8.7 wt% Co, 4.9 wt%Cr, 0.9 wt% Zr, 0.6 wt% Ta, 0.1 wt% O and 0.04 wt% Ti as processed.

At the outermost several microns of the bond coat 12a overlying thelayer 22 of "P" phase platinum aluminide is 35 a layer 23 of platinumenriched face center cubic gamma phase. The platinum enriched gammaphase for example has a composition of 33.4 wt% Co, 23 wt% Ni, 19.5 wt%Pt, 19.7 wt% Cr, 3.7 wt% Al, 0.3 wt% Zr and 0.2 wt% O as processed. Theplatinum enriched gamma phase may also contain platinum enriched gammaprime phase.

The yttrium is to be found in this platinum enriched gamma phase andalso to some extent in the "P" phase platinum aluminide. As discussedearlier during ageing oxygen diffusing through the ceramic coating 14and existing alumina layer 16a strips the aluminum from the surface ofthe platinum aluminide layer 22. This stripping of aluminum from theplatinum aluminide layer 22 causes the "P" phase platinum aluminide atthe surface of the platinum aluminide layer 22 to change to the platinumenriched gamma phase. Thus the alumina layer 16a is bonded to the "P"phase platinum aluminide layer 22 by a layer of platinum enriched gammaphase 23.

The "P" phase platinum aluminide layer 22 forms a continuous layer whichexhibits good stability together with the ability to reduce, or prevent,the migration of transition metal elements from the superalloy substrate10 and the bond coat 12a into the base of the ceramic top coat 14 byforming a continuous layer which blocks the migration of the transitionmetal elements. In particular the "P" phase platinum aluminide 22reduces the migration of titanium, hafnium and tantalum which are knownto decrease the adherence of the ceramic top coat 14 by promoting theformation of lesser protective oxides which grow at a faster rate thanpure alumina.

The platinum enriched alloy layer 20 comprises a platinum modifiedMCrAlY alloy layer 24 between the MCrAlY alloy layer 13 and the platinumaluminide layer 22. The platinum modified MCrAlY alloy layer 24comprises a platinum enriched gamma phase which slows down the migrationof the transition metal elements from the superalloy substrate 10 andthe bond coat 12a to the base of the ceramic top coat 14. Furthermorethe platinum forms strong complex molecules with free titanium andprevents the titanium migrating to the base of the ceramic top coat 14.The platinum modified MCrAlY layer 24 comprises a platinum enrichedgamma phase matrix with some "P" phase in the platinum enriched gammaphase matrix. The platinum enriched gamma phase for example comprises33.4 wt% Pt, 26.5 wt% Co, 17.7 wt% Ni, 16.5 wt Cr, 3.7 wt% Al, 0.9 wt%Zr, 0.8 wt% O, 0.4 wt% Y, 0.06 wt% Ta and 0.03 wt% W as processed. The"P" phase in this region for example comprises 59.7 wt% Pt, 15 wt% Ni,9.8 wt% Al, 8.5 wt% Co, 5.2 wt% Cr, 1.3 wt% Zr, 0.5 wt% O, 0.01 wt% Taand 0.01 wt% W as processed.

The layers 24 and 22 act to reduce, or prevent, the migration of harmfultransition metal elements to the platinum enriched gamma phase layer 23and the alumina layer 16a. The result is that the alumina is very pureand there is a very low growth rate of the alumina layer because thereis very little, or no, influence from metals which form oxides withhigher growth rates. Also the presence of yttrium in the platinumenriched gamma phase layer 23 is thought to influence oxide scaleadhesion by pegging of the alumina, eliminating voids at thealumina/bond coat interface and altering the alumina scale plasticity tolower the residual stress levels.

Our tests have shown that the "P" phase platinum aluminide layer 22 isvery stable to ageing at high temperatures, for example at temperaturesbetween 1100° C. and 1210° C. for 25 hours and still acts as a barrierto the movement of transition metal elements, titanium etc, towards theplatinum enriched gamma phase layer 23 and the alumina layer 16a becausevery low concentrations of the damaging transition metal elements aremaintained.

The ageing of the coating at temperatures between 1100° C. and 1210° C.causes the "P" phase composition in layer 22 to change between 36 wt%Pt, 27 wt% Ni, 14.8 wt% Co, 13.9 wt% Al, 6.3 wt% Cr, 0.6 wt% Ta, 0.4 wt%Zr, 0.07 wt% O, 0.03 wt% W and 0.02 wt% Ti at 1100° C. and 35 wt% Ni, 29wt% Pt, 14 wt% Co, 13 wt% Al, 6 wt% Cr, 0.9 wt% Ta, 0.5 wt% Zr, 0.4 wt%Ti, 0.3 wt% Y and 0.2 wt% O at 1210° C.

The "P" phase composition is broadly speaking 29-60 wt% Pt, 15-35 wt%Ni, 8-17 wt% Co, 9-15 wt% Al, 4.5-7 wt% Cr, 0-1 wt% Y and 0-2 wt% Ti,Ta, Zr, O and other elements.

The "P" phase is preferably formed in the temperature range of 1100° C.to 1200° C. As indicated in FIG. 4, the higher the tempera-cure of heattreatment the greater is the stability of the thermal barrier coating.At lower temperatures some "P" phase forms but it does not form acontinuous layer to block the migration of transition metal elements tothe alumina and platinum enriched gamma phase, particularly with thethinner thicknesses of platinum deposited.

Whilst the MCrAlY alloy of the bond coat has been disclosed herein indetail, it is only by way of example and may be substituted by otherknown aluminum-containing alloy bond coats, for instance, a nickelaluminide alloy composed of nickel and aluminum in stoichiometricproportions, such as that sold by the Chromalloy Corporation under thetrade name RT 69.

Application of the bond coat has been described as being by the vacuumplasma spray process in the Examples. However, other modes ofapplication could be used, e.g. argon-shrouded plasma spray or an EBPVDprocess of the type used for the ceramic top coat.

However, to obtain the best performance for a bond coat, if a MCrAlYalloy is used for the bond coat, it is preferred to use the vacuumplasma spray process to deposit the MCrAlY alloy onto the superalloysubstrate. The MCrAlY is preferably vacuum plasma sprayed, polished andpeened before the platinum-group metal is deposited onto the MCrAlY andheat treated. Our tests have shown improved adhesion of the ceramic topcoat when the bond coat is produced in this manner. It is theorized thisis because there is a higher percentage of free yttrium in the platinumenriched gamma phase and also in the "P" phase beneath the alumina layerdue to this method and the free yttrium forms yttria to enhance thealumina bonding. Alternative methods of deposition of the MCrAlY bondcoat include, but are not limited to, vacuum plasma spraying andsuperpolishing of the MCrAlY and electron beam deposition and peening ofthe MCrAlY.

EXAMPLE 7

In another experiment the effect of the method of depositing andprocessing of the MCrAlY was investigated and compared to MCrAlY bondcoats without platinum. On top of one MAR-M 002 specimen a MCrAlYcoating was deposited by electron beam deposition and was subsequentlypeened before the ceramic top coat was deposited. On another MAR-M 002specimen a MCrAlY coating was deposited by vacuum plasma spraying andwas subsequently superpolished before the ceramic top coat was depositedand on a further MAR-M 002 specimen a MCrAlY coating was deposited byvacuum plasma spraying and was subsequently polished and peened beforethe ceramic top coat was deposited. On another MAR-M 002 specimen aMCrAlY coating was deposited by electron beam deposition and wassubsequently peened before being plated with platinum, heat treated andcovered with a ceramic top coat. On specimens of MAR-M 002, RR2000,CMSX4 and CMSX10 MCrAlY coatings were deposited by vacuum plasmaspraying and were subsequently polished and peened before being platedwith platinum and heat treated and covered with ceramic top coats. Thesespecimens were cyclically heated to 1190° C. and maintained at 1190° C.for one hour and then cooled to room temperature. The time taken forthese coatings to spall at 1190° C. is shown in FIG. 6, and thisindicates that for the MCrAlY bond coatings the vacuum plasma spraying,polishing and peening produces the best result. Similarly for the MCrAlYbond coats with platinum diffused into the bond coats, the vacuum plasmaspraying, polishing and peening produces the best result.

Also of significance is the fact that the thermal barrier coating of theinvention does not adhere to the RR2000 alloy as well as it does to theother alloys. This is due to the high level of titanium, 4 wt%, in theRR2000 alloy. This also indicates the superiority of the thermal barriercoatings of the present invention relative to the thermal barriercoatings with a conventional MCrAlY bond coat. Note that the thermalbarrier coating of the present invention, as applied to MAR-M 002, CMSX4and CMSX10, have survived over 350 hours in this test and are stillintact.

Whereas in the above Examples 2, 4 and 6, only platinum was diffusedinto the outer layer of the alloy bond coat, other platinum group metalscould be substituted, such as palladium or rhodium.

EXAMPLE 8

In a further experiment the effect of the different platinum-groupmetals was investigated. On top of specimens of CMSX4 with MCrAlYcoatings different thicknesses of palladium were deposited at 2.5microns, 7.5 microns and 15 microns respectively, also 2.5 microns ofrhodium and 5 microns of platinum and a mixture of rhodium and platinum,2.5 microns of rhodium and 5 microns of palladium. These were heatTreated at the same temperature as the previous examples and alumina andceramic were deposited on them. Thereafter the specimens were cyclicallyheated to 1190° C. and maintained at 1190° C. for one hour and thencooled to room temperature. The time taken for these coatings to spallat 1190° C. is shown in FIG. 7, and this indicates that the platinumcoating gives the best performance.

EXAMPLE 9

In another experiment specimens of MAR-M 002 were coated with MCrAlY andthen ceramic as in Example 1. Other specimens of MAR-M 002 coated withMCrAlY, were then plated with platinum, aluminized at low temperature,800° C. to 950° C., and then coated with ceramic. Another set ofspecimens of MAR-M 002 were plated with platinum, aluminized at lowtemperature, 800° C. to 950° C., and then coated with ceramic. The lastset of specimens of MAR-M 002 were coated with MCrAlY, plated withplatinum, heat treated at 1100° C. to 1200° C. and coated with ceramicas in Example 2. The specimens were then tested to determine theiradhesive strength, critical load, and the results are shown in FIG. 8.The results show that the thermal barrier coating of the presentinvention is better than thermal barrier coatings using MCrAlY bondcoatings, MCrAlY with platinum aluminide bond coatings produced byplatinum aluminizing and platinum aluminide bond coatings produced byplatinum aluminizing. Platinum aluminizing the MCrAlY mentioned aboveforms a beta phase which is rich in (Pt,Co,Ni)Al and has much higheraluminum levels than that observed in the "P" phase produced in thepresent invention, also the platinum aluminide contains harmful elementsintroduced from the aluminizing pack, such as sulphur etc. It istheorized that the beta phase is not as stable as the "P" phase and thebeta phase breaks down to gamma and gamma prime phases more easily thanthe "P" phase. The beta phase and the "P" phase have different crystalstructures and it is theorized that it is the crystal structure whichdetermines the diffusion of elements through the phases, with the "P"phase blocking the diffusion of transition metal elements.

Although MAR-M 002 has been mentioned as an example of a superalloysubstrate to which the coatings of the present invention can besuccessfully applied, the invention should not be considered restrictedto use in conjunction with this specific alloy. For instance, thecoating has been successfully tested on a substrate of a nickel-basedalloy used for production of single crystal turbine blades, this alloybeing known as CMSX 4, produced by the Cannon-Muskegon Corporation of2875 Lincoln Street, Muskegon, Mich. 49443-0506, U.S.A. This alloy has anominal composition as shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        ELEMENT       WEIGHT %                                                        ______________________________________                                        Tungsten      6.4                                                             Cobalt        9.5                                                             Chromium      6.5                                                             Rhenium       3.0                                                             Aluminum      5.6                                                             Tantalum      6.5                                                             Titanium      1.0                                                             Hafnium       0.1                                                             Molybdenum    0.6                                                             Carbon        0.006                                                           Nickel        Balance                                                         ______________________________________                                    

The coating has also been successfully applied on a substrate of anickel based alloy used for production of single crystal blades, knownas CMSX10, also produced by Cannon-Muskegon Corporation. This alloy hasa nominal composition as shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        ELEMENT       WEIGHT %                                                        ______________________________________                                        Tungsten      5.5                                                             Cobalt        3.3                                                             Chromium      2.2                                                             Rhenium       6.2                                                             Aluminum      5.8                                                             Tantalum      8.3                                                             Titanium      0.2                                                             Molybdenum    0.4                                                             Niobium       0.1                                                             Nickel        Balance                                                         ______________________________________                                    

The coating has also been successfully applied on a substrate of anickel based alloy used for production of single crystal blades, knownas RR2000, produced by Rolls-Royce plc. This alloy has a nominalcomposition as shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        ELEMENT       WEIGHT %                                                        ______________________________________                                        Cobalt        15                                                              Chromium      10                                                              Aluminum      5.5                                                             Titanium      4                                                               Molybdenum    3                                                               Vanadium      1                                                               Carbon        0.015                                                           Nickel        Balance                                                         ______________________________________                                    

There is no reason to suppose that the invention could not also besuccessfully applied to cobalt-based superalloys.

Where the multi-layer coating of the present invention is applied toturbine blades or any other components, the precise thickness of eachlayer will be decided by experiment and/or calculation and will dependupon the temperature and corrosive agents to be experienced by thecomponents during operation.

We claim:
 1. A multi-layer thermal barrier coated article, comprising:asuperalloy substrate, an aluminum-containing alloy bond coat on thesubstrate, the aluminum-containing alloy bond coat having aplatinum-group metal enriched outer layer, the outer layer beingenriched with platinum-group metal relative to an inner portion of thealuminum-containing alloy bond coat, the platinum-group metal enrichedouter layer of the aluminum-containing alloy bond coat having a surfacelayer predominantly comprising a continuous layer of at least oneplatinum-group metal aluminide, an adherent oxide layer comprisingalumina and covering the platinum-group metal aluminide surface layer ofthe bond coat, and an insulating ceramic coating covering the oxidelayer, wherein the completed coated article has yet to be subjected tothermal stress.
 2. A thermal barrier coated article as claimed in claim1, wherein the aluminum content of the bond coating is in the range 5%to 40% by weight.
 3. A thermal barrier coated article as claimed inclaim 1, wherein the aluminum-containing alloy bond coating is selectedfrom the group consisting of nickel aluminide and cobalt aluminide.
 4. Athermal barrier coated article as claimed in claim 1, wherein thealuminum-containing alloy bond coating comprises an MCrAlY alloy, whereM is at least one of Ni, Co and Fe.
 5. A thermal barrier coated articleas claimed in claim 1, wherein the platinum-group metal enriched outerlayer of the aluminum-containing alloy bond coated article is enrichedwith platinum and the surface layer of at least one platinum-group metalaluminide predominantly comprises platinum aluminide.
 6. A thermalbarrier coated article as claimed in claim 1, wherein the oxide layercomprises at least 70% by volume of alumina.
 7. A thermal barrier coatedarticle as claimed in claim 6, wherein the oxide layer comprises atleast 90 vol. % of alumina.
 8. A thermal barrier coated article asclaimed in claim 7, wherein the oxide layer comprises at least 95 vol. %of alumina.
 9. A thermal barrier coated article as claimed in claim 1,wherein the ceramic coating comprises yttria stabilized zirconia.
 10. Athermal barrier coated article as claimed in claim 1, wherein theceramic coating has a columnar structure.
 11. A thermal barrier coatedarticle as claimed in claim 1, wherein the superalloy substratecomprises a nickel based superalloy.
 12. A thermal barrier coatedarticle as claimed in claim 1, wherein the superalloy substratecomprises a cobalt based superalloy.
 13. A thermal barrier coatedarticle as claimed in claim 5, wherein the surface layer of platinumaluminide contains at least 25 wt % platinum and at least 8 wt %aluminum.
 14. A thermal barrier coated article as claimed in claim 13wherein the surface layer of platinum aluminide contains at least 40 wt% platinum.
 15. A thermal barrier coated article as claimed in claim 14wherein the surface layer of platinum aluminide contains at least 50 wt% platinum.
 16. A thermal barrier coated article as claimed in claim 13,wherein the surface layer of platinum aluminide contains at least 10 wt% aluminum.
 17. A thermal barrier coated article as claimed in claim 1,wherein the bond coating comprises a platinum-group metal enriched gammaphase layer overlying the layer of at least one platinum-group metalaluminide, and the adherent oxide layer comprising alumina overlies theplatinum-group metal enriched gamma phase layer.
 18. A thermal barriercoated article as claimed in claim 1, wherein the layer of at least oneplatinum-group metal aluminide contains yttrium.
 19. A thermal barriercoated article as claimed in claim 17, wherein the layer ofplatinum-group metal enriched gamma phase contains yttrium.
 20. Athermal barrier coated article as claimed in claim 13 wherein the layerof platinum aluminide contains 29-60 wt % platinum, 15-35 wt % nickel,8-17 wt % cobalt, 9-15 wt % aluminum, 4.5-7.5 wt % chromium and 0-1 wt %yttrium.
 21. A thermal barrier coated article as claimed in claim 1,wherein the adherent oxide layer is sufficiently coherent to preventdiffusion of oxygen through the ceramic coating into the bond coating.22. A thermal barrier coated article as claimed in claim 1, wherein thecontinuous layer of at least one platinum-group metal aluminidecomprises 15 wt % or less of aluminum.
 23. A thermal barrier coatedarticle as claimed in claim 1, wherein the surface layer comprises aplatinum enriched gamma phase layer overlying a "P" phase platinumaluminide layer.
 24. A multi-layer thermal barrier coated article,comprising:a superalloy substrate, a bond coated article on thesuperalloy substrate, the bond coated article comprising analuminum-containing alloy coated article, a platinum-group metalenriched aluminum-containing alloy layer and a coated articlepredominantly comprising a continuous layer of at least oneplatinum-group metal aluminide, the platinum-group metal enrichedaluminum-containing alloy layer being enriched with platinum-group metalrelative to the aluminum-containing alloy coating, thealuminum-containing alloy coating overlying the superalloy substrate,the platinum-group metal enriched aluminum-containing alloy layeroverlying the aluminum-containing alloy coating and the coating of atleast one platinum-group metal aluminide overlying the platinum-groupmetal enriched aluminum-containing alloy layer, an adherent oxide layeroverlying the bond coating, the adherent oxide layer overlying thecoating of at least one platinum-group metal aluminide, the adherentoxide layer comprising alumina without other spinels in amountssufficient to substantially disrupt the alumina lattice structure, and aceramic insulating coating on the adherent oxide layer, wherein theplatinum-group metal enriched aluminum-containing alloy layer and thecoating of at least one platinum-group metal aluminide reduce migrationof transition metal elements through the bond coating to the ceramiccoating, and wherein the completed coated article has yet to besubjected to thermal stress.
 25. A multi-layer thermal barrier coatedarticle as claimed in claim 24, wherein the adherent oxide layer issufficiently coherent to prevent diffusion of oxygen through the ceramicinsulating coated article into the bond coating.